1. Field of the Invention
The present invention relates to a driver circuit, and particularly to a driver circuit having a CML circuit and a current setting circuit.
2. Description of the Related Art
A driver circuit has heretofore been used as an interface circuit for converting signals between different systems. The driver circuit is that for converting the swing amplitude of a differential input signal, and for outputting a differential signal. A conventional driver circuit is described, for example, in Japanese Patent Application Laid-open Publication No. 2004-350272 or in U.S. Pat. No. 6,429,700. FIG. 4 is a diagram showing a driver circuit described in JP-A No. 2004-350272.
A driver circuit described in JP-A No. 2004-350272 includes a current mode logic (hereinafter referred to as CML) circuit 10 having loads R1 and R2, transistors for switch M1 and M2, and a voltage control current source CS1. Furthermore, a level shift circuit 1 for supplying current is connected to the CML circuit 10. In this case, current is generated and supplied to the CML circuit 10 by the level shift circuit 1, and thereby the CML circuit 10 adjusts the amplitudes of output voltages outputted by output terminals of an OUT and an OUTB.
In addition, FIG. 5 is a diagram showing a conventional CML circuit 10 including loads R1 and R2, transistors for switch M1 and M2, and a voltage control current source CS1. In this case, a high level of an output voltage outputted by the CML circuit 10 is dropped at least by R5×Id from a power supply voltage VDD.
However, in the driver circuit described in JP-A No. 2004-350272, no description is given of a method for setting the value of a current supplied by the level shift circuit 1 to the CML circuit 10, and accordingly it is not clear how to determine the swing amplitude specifically. Furthermore, in the driver circuit described in U.S. Pat. No. 6,429,700, a high level of an output voltage outputted by the CML circuit 10 is dropped from a power supply voltage.
As described above, conventional driver circuits have difficulty in controlling an output voltage of a CML circuit.